Tantalum-titanium corrosion resistant alloy



limited States hatent Leland K. Lyons, Boulder City, Nevt, assignor toTitanium Metals Corporation of America, New York, N.Y., a

corporation of Delaware N Drawing. Filed June 25, 1959, Ser. No. 822,7273 Claims. (Cl. 75-174) This invention relates to corrosion resistantalloys and more particularly to an alloy containing tantalum andtitanium.

The corrosion resistant characteristics of tantalum metal are wellknown. It is employed, as a pure metal in various fabricated forms,under severe service conditions such as exposure to boiling acids forextended periods. To be useful in such applications, however, tantalummust be extremely pure inorder to provide ductile metal which can bereadily worked into bars, sheets, rods, tubes and other fabricatedforms. In addition, it has been found that many impurities, includingmetallic and nonmetallic elements andcornpounds, seriously reduce thecorrosion resistant properties of tantalum, often when present in onlysmall amounts.

Another factor affecting the usefulness of pure tantalum is itsextremely high cost whichis of the order of $50.00 to $80.00 per poundin fabricated form. Thus; While tantalum has valuable corrosionresistant properties its general usefulness has been severely restricteddue to its high cost. 7 7

It isa principal object of this invention to provide an improvedcorrosion resistant alloy. Another object of this invention is toprovide an alloy having corrosion re-' sistant properties approachingthat of pure tantalumbut at substantially less cost. Another object ofthis invention is to provide a. corrosion resistant and readilyfabricated tantalum, alloy.. These and other objects of this inventionwill be apparent from the following detailed description thereof.

In its broadest aspects this invention contemplates an alloy of tantalumand titanium in which the tantalum is present in amount more than 40% ofthe alloy, with the balance essentially all titanium. Preferably thealloy consists essentially of more than 40% and up to about 65%tantalum, balance substantially all titanium. An alternative embodimentof this invention contemplates the same type of preferred alloycomposition in which less than one-half of its. tantalum content isreplaced with columbium without substantial impairment of its corrosionresistant properties.

I have discovered that a loys of tantalum and-titanium containing, morethan 40% tantalum show remarkable corrosion resistant properties whichapproach those of pure tantalum. Table l which follows, shows thecorrosion in mils per year of alloys containing 50% and 60% tantalum,balance titanium, when immersed in. various extremely corrosive acids atboiling temperature;

Table I Corrosion RateMi1s per year 1 Gain due to oxide coating.

Patented Dec. 13, 11960 Pure tantalum has been found to have a corrosionrate" corrosion art a corrosion rate of less than 5 mils per year isconsidered resistant, and materials showing such corrosion resistancemay be and are, used in corresponding corrosive environments withanticipation in most applica-' tions of substantially unl'irnitedlife.

It is essential that the binary alloys of this invention contain morethan 40% tantalum. When 40% or less tantalum is present in the alloy, aninexplicable but nevertheless extremely sharp and critical reduction incorrosion resistance results. Table 2 following, shows the corrosioninmils per year of a 40% tantalum 60% titanium alloy and these resultsshould be compared with those shown in Table 1. It is evident thatreductions of the tantalum content to 40% has. resulted in a fortyfoldincrease in the corrosion rate in 20% HCl, a substantial increase in theH SO; corrosion rate, and at least atenfold increase in the corrosionrate in oxalic acid.

Table 2 Corrosion Rate-Mi1s:per year The alloy of this invention maycontain higher per- Eeritag'es of tantalum if desired, but the bestcombination of characteristics andcost areobtained when the tantalumcontent is within the preferred range of more than 40% and up to about.65% of the alloy. Since the price of tantalum powder is about $50.00per'pound and the price of titanium sponge is about $3.00 per pound,itwill be evident that the material cost, as an example, for a- 50% Ta,50% Tialloy will be only about $2650 orlitt-le' morethan one-half thatof pure tantalum; and such an alloy will have excellent corrosionresistant properties as shown above. It will also be' obviousthat as thetantalum content is increased the cost of the materials increases,approaching at the highest percentages that of pure tantalum. The upperlimit of 65% tantalum in thepreferred' range, represents a percentage atwhich the excelent properties of the allows are obtained at Veryreasonable cost. Binary alloys of titanium and tantalum containing morethan 60% and up to 65 are more particularly described and claimed in mycopending patent application Serial No. 40,561, filed July 5, 1960. Inaddition, tantalumtitanium alloy compositions containing m0re tl'ian40%'and up to about 65 tantalum are ductile and readily fabricated byforging, rolling, and other known working techniques, into bars, rods,sheets, plates, tubes. and other" fabricated forms for use in industry.At the same time, such alloys have relatively high tensile strength andmechanical properties which are more than adequate for use as structuralmaterials in corrosion environment applications.

A unique feature of the alloys of this invention containing more than40% and up to about 65% tantalum is that part of the tantalum contentthereof may be replaced With columbium, provided the replacement is lessthan one-half of the aggregate of these metals, without appreciableimpairment of their corrosion resistance and other properties. Table 3following, shows the corrosion rates of a 50% tantalum, 50% titaniumalloy in which various proportions of the tantalum have been replaced bycolumbium.

Table 3 Corrosion Rate-Mils per year The amount of columbium present inthe alloys shown in Table 3 above may readily be calculated as more thanabout 5% (for the alloy containing 2.9% Ob) and up to about 32% (for thealloy containing 16% Ch) of the aggregate amount of tantalum andcolumbium.

It will be evident from Table 3 that excellent corrosion resistance isshown by the alloys in which columbium is substituted in part fortantalum and these alloys also show mechanical properties and ease offabrication similar to those of alloys previously described.

Only less than one-half of the tantalum can be replaced with columbiumsince otherwise the corrosion properties will be adversely affected.Table 4 below shows the corrosion resistance of an alloy containing 25%tantalum, 25% columbium and 50% titanium exposed to the same conditionsdiscussed hereinbefore. It is clear that the corrosion resistance ofthis alloy, in which one-half the tantalum has been replaced bycolumbium, is not nearly as good as the resistance shown by the alloysin Table 3.

Table 4 Corrosion Rate-Mils per year The tolerance for columbium in thealloys of these embodiments of this invention is of the utmostsignificance. Tantalum and columbium often occur together in nature as acombined tantalite-combumbite ore. Tantalum and columbium, however, aredifficult to separate by chemical or metallurgical methods and one ofthe principal reasons for the high cost of pure tantalum is thedifliculty and expensive processing required to completely separate itfrom impurity columbium. Such pure tantalum is not essential in thealloys of this invention, as previously explained, and a much cheaperand more impure (with respect to columbium content) grade may beemployed. Provided the columbium content of the tantalum plus columbiumis less than 50%, such impure tantalum may be employed in the alloys ofthis invention without significant deleterious effect thereon, and at asubstantial cost saving even over the pure tantalumtitanium alloys.

A unique and most valuable property of alloys of this invention is theirlow specific gravity. Tantalum is heavy, having a specific gravity of16.6 as metal and 14.4 as powder. Titanium has a specific gravity of4.5. However, the specific gravity of the 50% Ta--50% Ti alloy, forexample, has been determined to be only 7.1. Therefore, there is ananomaly in that the specific gravities of these alloys are significantlyless than would be normally expected, considering the specific gravitiesof their constituent metals. Substitution of columbium, which has aspecifice gravity of 8.55, even further reduces the specific gravity ofthe alloys of this invention, as an example, an alloy comprising 41.5%Ta8.5% Co-50% Ti, has a specific gravity of only 6.9. These low specificgravities are of material importance, since the amount or volume perpound is obviously much greater. Thus, for example, ten pounds of puretantalum in sheet of inch thickness will amount to 0.92 square feet,while 10 pounds of the 41.5% Ta8.5% Cb50% Ti alloy in sheet of the samethickness will amount to 2.23 square feet.

Thus, this invention provides corrosion resistant alloys comparable inmany respects to pure tantalum at a small fraction of the cost.

Commercial grades of pure tantalum, columbium, and titanium, which willcontain incidental impurities in amount less than that which would causedeleterious embrittlement in the aloy or otherwise effect itsusefulness, are suitable for use to produce the alloys of thisinvention.

The alloys may be produced according to methods well known in the art,such as by melting together the constituent metals; for example,tantalum (or tantalum and columbium) powder and titanium powder orsubdivided sponge, may be admixed and compacted to form a consumableelectrode which is melted under vacuum in an arc furnace into aso-called cold mold to form an ingot. The ingot may be heated and forgedor otherwise wrought or cast into useful forms such as rods, tubes,sheets, plates, bars, and wire. Pure tantalum must be rolled or formedcold because of sensitivity to oxidation at elevated temperatures, butthe alloys of this invention containing up to about tantalum are much aless sensitive to this effect, and may be rolled or formed hot ifdesirable or convenient although a finish rolling cold will generallyproduce a better surface condition. These may be further fabricated toproduce, for example, tank and reactor linings, piping, and otherchemical plant apparatus. And as has been explained hereinbefore, suchproducts are useful in many applications where previously only puretantalum or more noble metals were known to have adequate corrosionresistance.

I claim:

1. A corrosion resistant alloy consisting essentially of more than 40%and up to about 65 in the aggregate of tantalum and columbium, with thecolumbium present in amount more than about 5% and less than one-halfthe aggregate amount of tantalum and columbium, balance substantiallyall titanium.

2. A corrosion resistant alloy consisting essentially of more than 40%and up to about 65% in the aggregate of tantalum and columbium, with thecolumbium present in amount more than about 5% and up to about 32% ofthe aggregate amount of tantalum and columbium, balance substantiallyall titanium.

3. A corrosion resistant alloy consisting essentially of about 34%tantalum and about 16% columbium, balance substantially all titanium.

References Cited in the file of this patent UNITED STATES PATENTS2,754.204 Jatfee et a1. July 10, 1956 2,819.960 V Bomberger Jan. 14,1958 2,922,714 Benham Jan. 26, 1960 FOREIGN PATENTS 718,822 Germany Mar.24, 1942 OTHER REFERENCES Journal of the Institute of Metals, voume 81,1952-53, article by Summers-Smith on pages 73-76.

1. A CORROSION RESISTANT ALLOY CONSISTING ESSENTIALLY OF MORE THAN 40%AND UP TO ABOUT 65% IN THE AGGREGATE OF TANTALUM AND COLUMBIUM, WITH THECOLUMBIUM PRESENT IN AMOUNT MORE THAN ABOUT 5% AND LESS THAN ONE-HALFTHE AGGREGATE AMOUNT OF TANTALUM AND COLUMBIUM, BALANCE SUBSTANTIALLYALL TITANIUM.